Amplitude modulation of microwaves



arch 20, QS

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R. T. GABLER ET AL1 AMPLITUDE MODULATION OF MICROWAVES Filed March 6, 1948 INVENTOR WITNESSES:

ATTORNEY Patented Mar. 20, 1951 Raymond T. Gabler, Inglewood, Calif.,

and

William Altar, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa.,v a corporation of Pennsylvania Application March 6, 1948, Serial No. 13,473

' 9. Claims. Y l

O'ur invention relates to modulation and has particular relationship to amplitude modulation of the oscillations in the microwave frequency range.

In accordance with the teachings of the prior art of which we are aware, modulation of such oscillations may be eiected by the application of a modulation voltage, to a reflector, an accelerating grid or any other electrode of an oscillator tube, such as a Klystron or magnetron. A disadvantage of modulation of this type is that undesirable frequency modulation results. Another disadvantage liesv in the sluggishness of the modulated output in failing toy follow with fidelity modulating voltages having high-frequency components, suchv vas* are encountered in tele-- vision and frequency modulation broadcast systems.

Modulation of microwave oscillation may also be eiected by coupling' a non-linear resistancel element, asa crystal, to an oscillator so that it absorbs the oscillations- The modulating voltage is' impressed on the element to vary its resistance'. A disadvantage of this scheme is that; the radiofrequencyv energy alosorbabl'el by a crystal or similar device is limited, and, therefore, thev modulated power output whi'ch can be derived by this expedient is limited. Y

It is accordingly van object of ourk invention to provide apparatus for producing amplitude modulation of high power oscillations of high frequencies, suchas microwave frequencies, with substantially no accompanying frequency modulation and with high delity. 1

Another object of our invention is toprovide a novel system of wave guides;

An incidental object of our invention is to produce a means for varying the impedance of a waveguide by the use of a modulated electron beam.

In accordance with our invention, we provide apparatus in which ther modulation is eiected" byl the cooperation of wave guides joined in a plurality of magic tee junctions to which certain of' the guides arecommon. Unmodulated microwave oscillations enter` the inputv branch of a main magic tee. The coeicients of reflections of twol conjugate branches of this magicv teeare varied in accordance with the modulations to bey impressed inauxiliary'magic tees of which these` branches constitute the respective input guides. These auxiliary magic tee junctions have variable impedances in their conjugate branches and a non-reflective load in the outputl branch. By varying theseimpedances; the kmagnitude 'of (C1, eea-56) energy being absorbed in the non-reflective loads:

The novel features that we consider character` istie rof our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together With additional objects and advantages thereof, will best be understood from thel following description of the specific embodiments when read in connection with the accompanying drawing, in which:

Figure ll is a view, partly in section and partly in perspective', of an embodiment of our inven-,

tion; and Y Fig.. 2 is` a sectional View, partly schematicr taken along liner Ill-II of Fig. l.

.The apparatus shown in. Figure' l comprises three systems of wave guides, of rectangular' transverseA cross-section, which are coupled to,- gether in what isf known in the art as, hybrid junctions or magic tees. At one end, an input? wave guide 4 is connected to fa source of oscillal tions 6, such as ahigh power Klystron, magnetron or resnatron. Aty the other end, the input wave guide 4l is connected through a main magictee junction 8 to a pair of conjugate branchesv 0. These conjugate branches are at right-angles to the input'wave guide Ilv and their sidesof greatest' area are parallel to the corresponding sides of the input waveguide. Connected' to the ends of the conjugate. wave guides It opposite to the main magicY tee junction throughv second and third magic tees l2 and I4 respectively, are pairs of conjugate impedance wave guides I6. The sid^s of greates area of the impedance wave guides itv are parallel to the corresponding sides of said conjugate wave guides l'0. 'Ih'ese impedance Wave guides I6 haveV coupled thereto, at their ends, variable impedances I8 to vary theimpedance of the Wave guides in accordance withV the modulation tobe impressed. In the preferred' practice ofl our invention, reactance tubesA are used" forthis purpose;

Also connected to each of the conjugate wave guides I8 through the second and third magic tees I2 and I4, is a load branch 2S containing a non-reflecting load 22. The load branches 2i) are perpendicular to the conjugate wave guides I8 and the impedance wave guides IS. The second magic tee junction i2 is one-quarter Wave length or an odd integral multiple quarter wave length further, as measured along the corresponding conjugate Wave guides It), from the electrical center of the main magic tee 8 than is the third magic tee I4.

Connected to the input wave guide 4 and the conjugate wave guides IS through the main magic tee 8, is an output wave guide 24 from which a load (not shown), such as an antenna, may be supplied. The output wave guide 24 is perpendicular to the other wave guides entering the junction, and the long dimension of its crosssection is parallel to the length of the input wave guide.

Oscillating energy from the source 8 ilows through input wave guide d to the main magic tee junction y8. On reaching this junction, the energy is deflected to the right and left toward the second and third magic tee junctions i2, I6. On reaching the junction of the second tee IQ, the` energy is deilected into the two impedance wave guides I6. The oscillations are reflected entirely from these impedance wave guides back toward the junction of the second magic tee IG.. The phase of these reflected waves will depend on the phase or" the modulating potential applied to the variable impedances I8 in these impedance wave guides. The signal potential is applied to the impedances in the impedance wave guides IE so as to cause the impedance of one impedance wave guide I6 to become more inductive while the impedance of the other impedance wave guide becomes more capacitive. This will result in a change in phase of the reflected waves. Thus, when the two waves meet at the junction of the second magic tee, they will have a component in phase and a component out of phase. That component of the reflected waves which is in phase will be deflected toward the nonreecting load. The rest of the energy of the oscillations will be deected toward the main magic tee. As the apparatus is symmetrical except for the quarterwave difference in length as measured along the two conjugate wave guides It connected to the main tee 8, the action of the third tee iti will be the same as the second tee I2 except that the oscillations arriving from the second tee will have traveled one-quarter wave length further than the oscillations from the third tee. The oscillations returning to the main tee will thus be substantially out of phase by 180. As these oscillations are out of phase by 180, and as the antenna wave guide presents a matched load to the potentials of the tee junction, substantially all of the energy will be deected out the antenna wave guide and none of the energy will be deflected back through the input wave guide toward the source of oscillations where it would produce undesirable frequency modulation.

The reactance tube, as employed in one embodiment of our invention, is shown in Fig. 2. The container comprises a cathode stem 28 which communicates with a, flanged cylindrical section 28. This cylindrical section 28 is designed to be inserted in the wave guide. The walls of the cylindrical section 28 must be substantially of a non-conducting material so the electromagnetic oscillations will not be reflected. In the interior of the tube, we have a heated cathode 30, a control grid 32, two accelerating grids 34, a resonant cavity 36 and a plate or anode 38.

The electrons ejected from the cathode 30 either return to the cathode 38 or proceed past the control grid 32 depending on the phase of the modulating voltages impressed on the control grid. Those electrons passing the control grid 32 are accelerated by the accelerating grids 34, pass through the cavity 36 and impinge on the plate 38.

The operation of the reactance tubes I8 depends on the detuning of a resonant cavity by a variable electron beam. The intensity of the beam can be varied by a grid. As the resonance frequency of the tube varies, the cavity represents a varying reactive element at the operating frequency capable of covering a wide range of capacity and inductive susceptance values as a function of the beam intensity or of the grid voltage. Using these tubes, we me able to vary susceptances quickly and with practically no dissipation of power. The use of these tubes is not to be considered as the only method of Varying the ref actance of the wave guides.

where Yza and Ya) are susceptances of the two impedance wave guides connected to the second magic tee, and Yaa and Yeh are the susceptances oi' the two impedance wave guides connected to the third magic tee.

The amplitude of the wave reflected toward the source of oscillations is given by TVi-Ta Y R- 2 (2) where r2 and r3 are the reflection coefcients in the two conjugate wave guides which lead from the magic tee to the second and third tees, the magnitude of the wave entering one of the conjugate branches being unity. In view of the symmetry of the arrangement except for a quarter-wave difference in length of the side branches, these two reflection ooecients are equaland opposite in sign so that the generator is presented with a matched load, no matter what the instantaneous susceptance values of the reactance tubes are, so long as they obey the equations (1) The transmission coeilcient representing amplitude and phase of the wave leaving the antenna is given by These equations have been given by way of explanation and are not meant to restrict the operation of this invention. This specication discloses Wave guides as a means of transmission and a magic tee as the hybrid junction. Such facilities are included in accordance with the specic aspects of our invention. It must be understood, however, that any of several types of hybrid junctions employing transmission lines consistent with the nature of said junctions can be used in accordance 'with the broader aspects of our invention. Accordingly, coaxial lines, conductor lines, or any other conductors suitable for the operating frequency, may be utilized.

Although we have shown and described certain specific embodiments of our invention, we are fully aware that many modications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. In combination, a first magic tee having four branches, a source of oscillations connected to a first of said branches, a second magic tee connected to a second of said branches and a third magic tee connected to a third of said branches, said second magic tee being onequarter wave length further from the junction of said first magic tee than is said third magic tee, said second and said third magic tees having variable impedances on two of their branches and a non-reflective load on a third branch of said second and said third magic tees, a fourth branch connected to said first magic tee containing a load for deriving the net energy produced therefrom.

2. In combination, a first magic tee having four branches, a source of oscillations connected to a rst of said branches, a second magic tee connected to a second of said branches and a third magic tee connected to a third of said branches, said second magic tee being onequarter wave length further from the junction of said first magic tee than is said third magic tee, said second and said third magic tees having reactance tubes on two of their branches and a non-reflective load on a third branch of said second and said third magic tees, a fourth branch connected to said first magic tee containing a load for deriving the net energy produced therefrom.

3. In combination, a first magic tee having four branches, a Klystron oscillator connected to 'a first of said branches, a second magic tee connected to a second of said branches and a third magic tee connected to a third of said branches, said second magic tee being one-quarter wave length further from the junction of said first magic tee than is said third magic tee, said second and said third magic tees having variable impedances on two of their branches and a nonreective load on a third branch of said second and said third magic tees, a fourth branch connected to said first magic tee containing a load for deriving the net energy produced therefrom.

4. In combination, a first magic tee having four branches, a Klystron oscillator connected to a first of said branches, a second magic tee connected to a second of said branches and a third magic tee connected to a third of said branches, said second magic tee being onequarter wave length further from the junction of said first magic tee than is said third magic tee, said second and said third magic tees having reactance tubes on two of their branches and a non-reective load on a third branch of said second and said third magic tees, a fourth branch connected to said first magic tee containing a load for deriving the net energy produced therefrom.

5. In combination, a first hybrid junction having four conductive paths, a source of oscillations connected to a first of said conductive Y paths, a second hybrid junction connected to a second of said conductive paths and a third hybrid junction connected to a third of said conductive paths, said second hybrid junction being one-quarter wave length further from the junction of said first hybrid junction than is said third hybrid junction, said second and said third hybrid junctions having variable impedances on two of their conductive paths and a non-refiective load on a third conductive path of said second and said third hybrid junctions, a fourth conductive path connected to said first hybrid juncti-on containing a load for deriving the net energy produced therefrom.

6. Device for varying the reflection coefficient of a wave guide comprising a hybrid junction connected to said wave guide, two branches of variable impedance connected to said hybrid junction, and a fourth branch connected to said hybrid junction containing a non-reflecting load.

7. In combination, a hybrid circuit having four branches, a source of oscillation connected to an input branch, two substantially reactive variable elements connected to a second and third of said branches, means for varying the reactive elements in a coordinated manner such that the sum of all waves entering a fourth branch varies in a predetermined manner while the sum of all waves reflected through said first branch toward said source remains essentially constant.

8. As an article of manufacture, a first hybrid junction including an input wave guide, an output wave guide and a pair of conjugate wave guides, a second hybrid junction coupled to one of said conjugate wave guides and a third hybrid junction coupled to the other of said coniugate Wave guides, said conjugate wave guides each constituting the input wave guide of its corresponding hybrid junction and said second and third wave guides each having in addition output wave guides and conjugate wave guides.

9. An article according to claim 8 characterized by the fact that the second and third hybrid junctions are disposed at distances differing by an odd integral number of quarter wave lengths of the frequency of the oscillations for which the guides are designed from the electrical center of the first hybrid junction along the corresponding conjugate wave guides.

RAYMOND T. GABLER. WILLIAM ALTAR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,421,725 yStewart June 3, 1947 2,436,828 Ring Mar. 2, 1948 2,438,768 Stewart Mar. 30, 1948 

